A typical down range surveillance scene is not a single point in space, but is usually a horizontal panorama; one which requires sufficient illumination across its entire spread for optimal imaging. Also, most surveillance illumination has a vertical aspect that goes from foreground to background, so that a majority of its illumination must strike a target area that is angled below the illuminator mounting position, such as a pole or structure. A typical single illumination source delivers light with a circular Gaussian distribution, namely a central bright spot which then rapidly diminishes in intensity the further one is from its central axis. Since typical surveillance imagers are wide-angle in coverage, they require a wide-angle source which delivers uniform illumination across the entire imaging area, or target area. To one skilled in the art, it is clear that uniform illumination on a target area is an important aspect of image quality and one which allows the camera to extract the most information from a scene.
Due to the inverse square law of illumination, the intensity of light falling on a target area decreases in proportion with the square of the distance. For wide-angle illumination, distance to the middle of the horizontal area is considerably less than the distance to the edges. In order to compensate for the increased distance an illuminator must produce a horizontal intensity profile with strong bias towards these edges. The standard Gaussian illuminator intensity profile has the maximum at the center of the beam which is the opposite of that needed to create a uniform irradiance pattern over a target area. When using a camera and wide-angle lens arrangement to view a typical surveillance scene, a certain proportion of the image contains light traveling in the foreground, i.e., to the target area, and a certain amount that actually illuminates the target area. An optimal surveillance illumination system should deliver the maximum illumination evenly across the target area with minimal wasted energy falling outside the target area.
In addition, it is well known that a camera's dynamic range at night is determined by the range of signal it can pick up from the scene, and this ranges from the brightest object to darkest object the camera can detect. Under low light situations, or at night, the camera control algorithm will determine optimal parameters of exposure and gain, striving to maintain the highest quality image with the most dynamic range and therefore the most information in the image.
An ineffective illumination system with a peak to minimum ratio of 10:1 has a drastic impact on the usable information that can be extracted from the scene. A typical camera has 30-50 dB of dynamic range, and extended dynamic range cameras are being introduced to the market with dynamic range of 60 db to over 100 dB in some lighting conditions. Lighting can have the effect of reducing the effective dynamic range by 5-10 dB which is enough to make an ordinary camera with good illumination outperform an expensive high dynamic range camera with poor illumination.
The ideal situation is to combine a high dynamic range camera with an illumination system that provides very even illumination so the entire dynamic range of the camera can be utilized in extracting information from the scene instead of compensating for lighting irregularities.
Illumination systems that produce a circularly diverging beam require the installer to point the peak of the beam at the farthest target point. For a fixed target distance there is an optimum beam intensity profile in the vertical orientation. When viewing at the same distance with a wider and wider view and matching the circular illumination the illuminator moves further away from the optimum in the vertical orientation and wastes more light. Moreover, as the peak of the light source is pointed above the line of the target, a large proportion of the light is above the target area and is not utilized.
Microdiffractive light shaping diffuser (LSD) technology (WO2008037049) is a significant improvement in full-scene wide-area illumination by delivering light with an asymmetric Gaussian distribution enabling more even illumination on the target area, and less wasted light in the vertical plane than circular Gaussian sources. In spite of the elliptical distribution of LSD illuminators, they have similar energy waste outside the primary illuminated area, as with its circular distribution brethren.
Most video surveillance applications have the goal of achieving a uniform video signal for a particular target throughout a scene. Much effort has been made to increase sensitivity, improve spectral response to generate more signal with less light but little work has been done on imaging systems to actually produce a uniform image throughout the field of view. An active illumination imaging system is made up of the illuminator, camera and lens in the simplest form. It is important to understand that all three elements: the illuminator as well as camera lens and sensor have significant impact on the video signal produced.
A well known imaging phenomenon called vignetting describes the drop off of video signal from the center to edges of a scene. The result is that even under uniform illumination throughout a target area a sensor will produce a much higher signal in the middle than on the edges of the image. This effect is pronounced for wide angle views and low light situations where large aperture lens is often used.
With the prior state of the art surveillance illuminator technology, the illumination varies from the middle to edges of the scene by 20% while the video signal can have a difference of 100% of the video signal for the same target. Even with uniform illumination across a scene a difference of up to 35% of the video signal is observed for the same target in common situations.